1. Field of the Invention
The present invention relates to a method of controlling an automatic transmission comprising a plurality of gear trains, and more particularly to a method of controlling upshifts in such an automatic transmission.
2. Description of the Prior Art
Various automatic transmissions with a plurality of gear trains are known in the art. One such automatic transmission comprises a planetary gear automatic transmission as disclosed in Japanese laid-open patent publication No. 62-137457.
The disclosed planetary gear automatic transmission has, between input and output shafts, a plurality of power transmitting paths composed of combinations of planetary gear trains, and a plurality of engaging means (clutches and brakes) for selecting one of the power transmitting paths for establishing a desired gear position. The gear positions of the automatic transmission are controlled by controlling operation of the engaging means.
In such planetary gear automatic transmissions, the gear positions are mostly established by engaging and disengaging the engaging means. It is sometimes necessary to engage and disengage a plurality of engaging means simultaneously for changing gear positions. For example, for shifting from one gear position to another gear position over two or more gear positions, the planetary gear automatic transmission disclosed in the above publication is required to disengage two engaging means and engage two other engaging means.
Since controlling such simultaneously engagement and disengagement of plural engaging means is quite complex, when a shift command is issued to shift from one gear position to another gear position over two or more gear positions, the disclosed planetary gear automatic transmission actually operates to successively go through, or, to establish, the intermediate gear positions until the target gear position is reached, rather than skipping from the present gear position directly to the target gear position. In the disclosed planetary gear automatic transmission, one of the plural engaging means which are engaged in adjacent gear positions serves as a common engaging means. Thus, when the transmission goes successively through a plurality of gear positions, only one engaging means may be disengaged and only one engaging means may be engaged to establish each of the gear positions. As a result, the transmission control process is relatively simple.
According to the above transmission control process, inasmuch as the intermediate gear positions are actually established one by one when the transmission is shifted from the present gear position to the target gear position, the acceleration of the automobile which incorporates the automatic transmission and the rotational speed of the engine combined with the automatic transmission are rendered irregular or discontinuous at the time the intermediate gear positions are established. Therefore, the driver or passengers of the automobile tend to feel awkward because of such acceleration or engine speed irregularities.
It is possible to shift from the present gear position directly to the target gear position while skipping the intermediate gear positions. For such shifting, it is necessary to simultaneously disengage the two engaging means which are engaged to establish the present gear position and to simultaneously engage the two engaging means which are engaged to establish the target gear position. Since these engaging means should be disengaged and engaged in precise synchronism, the rotational speeds of rotating members in the transmission need to be detected accurately and complex control procedures are required. Therefore, the overall control process is highly complicated and difficult to achieve.
Particularly, a certain gear position can be established only when two engaging means are engaged. Consequently, if one of the engaging means is engaged out of synchronism with the other engaging means, then the transmission is liable to be held in an unwanted neutral condition allowing the engine to race, or gears of two power transmitting paths are apt to mesh simultaneously.